This invention relates to a particulate trap for collecting and removing particulates such as carbon contained in exhaust gas discharged from a diesel engine.
Exhaust gas discharged from car engine is one of the major causes of air pollution. It is therefore important to develop techniques for removing harmful components contained in exhaust gas.
It is especially important to remove particulates contained in the exhaust gas discharged from diesel engines, which are mainly NOx and carbon.
Heretofore, various efforts have been made to remove such harmful components before the exhaust is discharged from the engine. Such efforts include putting EGR (exhaust gas recirculation) on the engine and improving a fuel injector and the shape of a combustion chamber. However none of these measures is a definitive solution. Another known measure is to provide an exhaust trap in the exhaust passage to collect the particulates contained in the exhaust (as proposed in Unexamined Japanese Patent Publication 58-51235). This method in which the exhaust is treated after being discharged from the engine, is considered more practical and has been studied vigorously.
Such a particulate trap for collecting particulates in exhaust gas has to satisfy the following requirements in view of the conditions of use.
First, it has to be capable of collecting particulates with such a high efficiency that the exhaust is cleaned sufficiently. Each country sets by law a different upper limit of the particulate emissions. For example, the Japanese government has set a long-term target of upper limit at 0.08 g/Km in the 10-mode test in passenger cars, light trucks and buses, which is to be attained by the year 2000. In the U.S., the 1994 EPA restriction has set such an upper limit at 0.1 g/HP.Hr for heavy-duty cars in the transient mode and at 0.08 g/mile for light-duty cars in the LA-4 mode. These are pretty tough regulations. The amount of particulates contained in the exhaust depends on the displacement of the diesel engine and the load applied. However it is generally considered that such a trap has to be capable of collecting, on the average, 60% or more of the particulates in the exhaust to meet such regulations.
Secondly, the pressure drop of exhaust gas must not be so large. As the exhaust gas passes through the trap and the particulates therein are collected by the trap, its resistance to gas flow increases. If the resistance is too great, an undesirable back pressure will act on the engine. It is therefore necessary to restrict such back pressure below 30 KPa after collecting particulates. For this purpose, it is necessary that not only the initial pressure drop be sufficiently low, but the resistance to the flow of exhaust be small, i.e. the pressure drop does not rise too much even after the particulates have been collected.
Furthermore, it is necessary to periodically remove the collected particulates when a predetermined amount of particulates has been collected to regenerate the trap so that it can regain its initial particulate collection capacity. Thus, the third requirement for a particulate trap is that it is tough enough to endure repeated regeneration treatments. It is considered the most promising regeneration method to burn particulates with an electric heater or a burner. In either case, the collected particulates have to be heated to a temperature higher than the ignition point of the particulates (usually 600.degree. C.). Recycling treatment, i.e. the burning of the collected particulates, has to be completed before the back pressure increases to such a degree that the engine performance drops or the drivability worsens. After regeneration, particulates are collected again. Trappings and regenerations are repeated. Thus, the pressure drop is always kept at a level below a predetermined value. It is therefore important that the filter element be made of a material which can not only sufficiently withstand the repeated regeneration treatments but show sufficiently high resistance to corrosion by the atmospheric gas contained in the exhaust.
The above requirements are met if a honeycomb-shaped porous member made of cordierite ceramic is used as the filter member in the particulate trap. Such a filter is also considered most practical. However with this conventional arrangement, in which the particulates collected by the cordierite ceramic are burned, the filter is repeatedly heated to a rather high temperature. Thus, the filter frequently melted or developed cracks due to thermal shock when the filter was heated for regeneration and then cooled down.
Because this type of filter element has a problem in that the regeneration control is extremely difficult, it is not practically used yet.